AArch64: Fix hwasan failure in readline.

My previous fix added an unchecked call to fgets in the new function readline.
fgets can fail when there's an error reading the file in which case it returns
NULL.  It also returns NULL when the next character is EOF.

The EOF case is already covered by the existing code but the error case isn't.
This fixes it by returning the empty string on error.

Also I now use strnlen instead of strlen to make sure we never read outside the
buffer.

This was flagged by Matthew Malcomson during his hwasan work.

gcc/ChangeLog:

	* config/aarch64/driver-aarch64.c (readline): Check return value fgets.

(cherry picked from commit 341573406b392f4d57e052ce22f80e85a7c479e9)

diff --git a/gcc/config/aarch64/driver-aarch64.c b/gcc/config/aarch64/driver-aarch64.c
index 1d1d99f72082d8bec8584081899ff6507fcb670b..106dc00e9dc14cc3ee2ad96504868ca824bf28f4 100644 (file)
--- a/gcc/config/aarch64/driver-aarch64.c
+++ b/gcc/config/aarch64/driver-aarch64.c
@@ -194,10 +194,16 @@ readline (FILE *f)
       size += buf_size;
       buf = (char*) xrealloc (buf, size);
       gcc_assert (buf);
-      fgets (buf + last, buf_size, f);
+      /* If fgets fails it returns NULL, but if it reaches EOF
+        with 0 characters read it also returns EOF.  However
+        the condition on the loop would have broken out of the
+        loop in that case,  and if we are in the first iteration
+        then the empty string is the correct thing to return.  */
+      if (!fgets (buf + last, buf_size, f))
+       return std::string ();
       /* If we're not at the end of the line then override the
         \0 added by fgets.  */
-      last = strlen (buf) - 1;
+      last = strnlen (buf, size) - 1;
     }
   while (!feof (f) && buf[last] != '\n');